Metal-organic frameworks (MOFs) constitute a rapidly growing class of solid-state compounds. They are built up from multitopic organic linkers and metal-based nodes which are interconnected via coordination bonds. From a functional materials perspective, due to their chemical diversity and high surface area, MOFs have garnered tremendous interest for many practical applications including gas storage and separation, chemical catalysis, sensing, conductivity, and light harvesting. Given the highly modular nature of MOFs, the introduction of chemical functionality should be straightforward (at least in comparison to many other solid-state materials). Unfortunately, de novo syntheses (i.e., one-pot solvothermal syntheses) often encounter problems associated with linker solubility, linker stability, and/or the formation of undesirable structures or side products (e.g., the coordination of metal ions to the functionalized linker).
Post-synthesis incorporation of desired functionality within a given MOF structure has proven to be a key strategy in overcoming many synthetic challenges associated with de novo MOF preparation. Some of the most attractive strategies include functionalization at the metal node (via dative bonding), covalent modification of the organic linker, and solvent-assisted linker exchange (SALE) which involves exchanging one organic linker for another.
In the context of carbon capture and sequestration (CCS), fluorinated MOFs have recently emerged as attractive candidates given their hydrophobicity and the presence of X—F dipoles (where, for example, X can be C, P, Si). Hydrophobicity should render the MOF stable towards water vapor, a component in post-combustion CO2 capture, while the presence of C—F dipoles should lead to favorable interactions with the quadrupole of CO2 (i.e., high isosteric heats of adsorption, Qst). For example, Eddaoudi has shown an enhancement of CO2 adsorption in MOFs containing C—F dipoles in the linker; Qst0 values as high as 60 kJ/mol have been observed. Likewise, MOFs constructed with pyrazine and bipyridine linkers that utilize anionic hexafluorophosphate and hexafluorosilicate as pillars have demonstrated high selectivity for CO2 with moderate to high Qst0 values (31-45 kJ/mol).